The problem addressed by this invention is encountered in oscillator circuits used for generating precision clock signals. Precision clock signals are widely used in digital circuits to synchronize the activities of one digital circuit to the rest of the circuits in the system. A digital system may include one or more microprocessors, bus controllers, peripheral controllers, memory, and peripherals, such as disk drives, all of which may typically be synchronized from one system clock. A less complex system may include a single microcontroller with at least one input device, such as a sensor, and at least one output device, such as an ignition controller. And yet a less complex system may include an oscillator to provide a frequency reference for the speedometer or tachometer of an automobile. It will be appreciated that precision oscillators are used in a full range of simple to complex systems.
Referring now to FIG. 1, a precision oscillator 24 according to the prior art is shown. The oscillation cycle begins by setting and resetting RS flip-flop 20. The flip-flop 20 is set by the current source 2 charging the capacitor 4. As the capacitor 4 is charged, the voltage on the non-inverting input of the comparator 8 rises. When this voltage exceeds the voltage, Vcalib, on the inverting input, the output of comparator 8 will rise to the positive voltage rail of the comparator thereby setting the output of the flip-flop 20 to a digital "1" voltage. This subsequently turns off the current source 2, turns on the transistor 6, turns off the transistor 14 (which was on), and turns on the second current source 10. Consequently, the second capacitor 12 is charged by the second current source 10 and the voltage on the non-invert input of the second comparator 16 rises. When the voltage on the non-inverting input of the second comparator 16 exceeds the voltage on the inverting input, the output of the comparator will swing to the positive voltage rail thereby resetting the output of the flip-flop 20 to a digital "0". The resetting of the flip-flop 20 subsequently turns on the first current source 2, turns transistor 6 off, transistor 14 on, and turns the second current source 10 off, thereby restarting the oscillation cycle.
Although the illustrated prior art circuit 24 does not require a precision crystal, it is still relatively complicated in that the circuit requires two capacitors 4 and 12, two comparators 8 and 16, and an RS flip-flop 20. In addition, precise calibration for a duty cycle of the oscillator may be difficult to achieve.